A filter device is used as a glare-protection device, for example, in protective welders' masks, welders' helmets or welders' goggles. Such a device comprises a light-protection filter with at least one active optical filter element, an electronic device for controlling the at least one active optical filter element, a light sensor working together with the electronic device, an electric power supply, in particular a solar cell, for the electronic device and the at least one active filter element.
Glare-protection devices of this type are known, for example from WO 96/10767 and EP-A-0 678 288. Usually they contain a liquid crystal cell (LC cell) as the active filter element, which more or less blocks the penetration of light as soon as the light sensor is acted upon by light past a preset intensity threshold. Such filter devices have many uses, a typical example is the use as windows for protective welders' masks, welders' helmets or welders' goggles.
The filter devices described in the mentioned publications are designed as so-called "monoblocs", i.e. they physically consist of a single component containing all functional elements of the filter device. Here, the functional elements have been encapsulated in a thin plate and in this way are protected from the effects of the surroundings. Because of the design as a thin, plate-shaped monobloc, the physical compatibility with the common passive light-protection filters (dark glasses) is achieved, so that the filter device can be directly exchanged against such passive light-protection filters. By means of this it is possible to retrofit glare-protection devices equipped with passive light-protection filters in a simple way by means of the more effective and more advantageous active electro-optical filter devices.
However, designing the filter in the form of a monobloc has the important economical disadvantage, also doubtful for reasons of protecting the environment, that the filter device as a whole must always be replaced, even if only one of its functional elements fails. Because of the encapsulated structure, a repair of the functional elements is as good as impossible.